The World Energy Outlook in 2011 estimated that the global energy needs will increase with 75% by 2035, and over 1 trillion US dollars is needed for large-scale investment for future energy supply in coal and biofuels (Statistics SA. 2011). The South African Government has also revised its own energy strategy so as to clarify government policy regarding the country's supply and consumption of energy for the foreseeable future. The South African Energy Policy White paper (the latest energy policy) advocates continuous deregulation, maintaining a coal resource database, promotion of low-smoke coals for households, promotion of end-use efficiency and clean-coal technologies, investigating the use of coal-bed methane, and the use of discard coal (White Energy Company. 2012).
Of the 255 Mt/a of coal produced in South Africa, 60 Mt/a of coal is discarded as fine deposits from mining and transport operations because of its perceived poor quality (high ash and sulphur) and volatiles diminish as stockpiles age (Wagner, N. J. 2008 and Bunt, J R et al. 1997). Coal is also discarded due to size, i.e. coal fines are generally classified as particles <500 microns that are separated from the coal during the beneficiation process (UNFCCC. 2001). In Coal-To-Liquid plants (CTL) as example, fixed-bed gasifiers use a lower cut size of 5 to 8 mm in the process for the production of liquid fuels, thereby delivering a higher percentage of “fine” coal, which is mainly used in the generation of high pressure steam (in pf combustion plants) for use in the gasification process (Ratafia-Brown, J. 2002). However, there is an overbalance of fine coal in the circuit, which inevitably leads to coal being discarded into storage ponds and slimes dams (EUBA. 2007).
Due to the importance of coal as a non-renewable resource these coal fines can be utilised. Discard coal having a heating value of c.a. 16 MJ/kg can be burnt by Eskom, making it a viable energy source; can be beneficiated and added back to a washed coarser product where contracts allow; or could be gasified in fluidised-bed and entrained flow reactors, but these technologies are extremely capital expensive and to date have not been implemented in South Africa (Radloff, B et al. 2004 and Hippo, E. J., et al. 1986). The Highveld coalfields are of great importance to the long-term life of Sasol's Synthetic Fuels (SSF) Division, but these coalfields are close to exhaustion, with an estimated remaining recoverable coal reserve of 9 billion tans (Sharma A, et al. 2008). Utilisation of discarded Highveld fines as example will assist in increasing the coal reserves available to Sasol, and will also be aligned with Government policy. An alternative process should therefore be found to enable coal to be used more effectively, thereby improving the life span of coal resources for future generations (UNFCCC. 2001).
Agglomeration of fine coal can be classified into briquetting and pelletizing, either with or without binders. Pellets are normally cylindrical with a diameter ranging from 6 to 12 mm and a length of 4 to 5 times the diameter. Briquettes can also be cylindrical with a diameter of 80 to 90 mm, or parallel-piped with average dimensions of 150×70×60 mm (Eakman, J. M. 1980). Briquetting with a binder has shown success in Australia (Wallerawang Colliery) where 70 kt/yr. of 50 mm diameter briquettes (10-20% moisture content) is produced by a double roller press for the production of fuel for use in a conventional power station using fine coal washery rejects. The binder used in this process is however unknown (Nel, S. 2013).
From a South African context, a process was developed by Mangena and de Korte (Suzuki, T. 1984) to transform discarded South African ultra-fine coal into valuable low-smoke fuel, which may be supplied to the domestic market at a price similar to that of coal. It was concluded that if it was accepted by the end-users, this fuel could help to reduce the amount of localised air pollution in households, particularly in the impoverished townships. Mangena & du Cann (Takaranda, T. 1986) also studied pillow-shaped binderless briquettes produced with a Komarek B-100A roll-type briquetting process using South African coal. This machine has a roll diameter of 130 mm and a maximum allowable pressure of 17 MPa. The briquettes produced had the following measurements: 40 mm (length)×19 mm (breadth)×13 mm (thickness). Their study concluded that briquetting with fresh, vitrinite-rich coking and blend coking coals were the most successful. Other alternatives should however be tested because of the effect that weathering had on the amenability of coal to binderless briquetting. The negative influence could be caused by the kaolinite content in the weathered coal. Binderless briquetting is however a possible utilization option for discard coal (Takaranda, T. 1986).
The Sasol-Lurgi fixed-bed dry-bottom gasification process, as deployed in the Sasol Synfuels plant in Secunda, South Africa, consumes more than 30 million tons of coal per annum. This coal is used for gasification in the 84 gasifiers in the plant, which produces more than 150 000 barrels per day of oil equivalent of fuels and chemicals. The feed to the Sasol-Lurgi gasifiers consists mainly of coarse (>6 mm) low-rank bituminous coal and extraneous rock fragments (usually carbonaceous shale, siltstone, sandstone, and mudstone). This feedstock is processed in the gasifiers at elevated temperatures (up to 1350° C.) and pressures less than 30 bar to produce synthesis gas (also referred to as syngas), which is a mixture of carbon monoxide and hydrogen. Coal gasification ash is a major by-product of the gasification process. This ash, referred to as “coarse ash”, is a combination of red and white to grey sintered clinkers with heterogeneous texture varying from fine material to large irregularly shaped aggregates of sizes ranging from 4 to 75 mm. During the Sasol-Lurgi gasification process, the coal is gasified in counter-current mode to the gasification agent, i.e. steam and oxygen at an inlet temperature of approximately 350° C. The coal enters the gasifier through the automatically operated coal lock on top of the gasifier and is gasified by the steam and oxygen that are introduced at the bottom of the gasifier while gravitating down the moving bed. The coal is exposed to the different reaction zones in the gasifier, i.e. the drying and the pyrolysis zone where drying and devolatilization takes place. After this zone, the char enters the gasification zone where different gasification reactions take place. The gasification zone is followed by the combustion zone where the combustion of the char is affected in the presence of oxygen. The ash zone is the last zone where the ash is cooled down by the excess steam and oxygen that is fed in at the bottom of the gasifier. Sulphur is introduced to the Sasol-Lurgi gasification plant with the coal, where it is bonded either in the organic matter or mineral matter of the coal (Skhonde, P. 2009).
Sulphur in coal is derived primarily from two sources: original plant materials and inorganic materials in the coal-forming environment. Abundance of sulphur in coal is controlled by the depositional environment and the digenetic history of the coal seams and overlying strata. The seawater interaction with peat results in elevated levels of sulphur in coals. The sulphur in low-sulphur coals is derived only from plant materials. The amount and forms of sulphur vary among different coals due to the differences in the coal formation processes and the ranks of coal. South African coals that are used for the Sasol-Lurgi gasification process are normally low-grade medium rank C (bituminous) coal with a total sulphur content of approximately 1-2 weight percentage (wt. %), on an as-received basis. This value of the sulphur content is very low compared to some of the coals that are used worldwide for combustion and other processes. In a Ph.D. thesis by Benson (Benson, S. A. Ph.D. Thesis, 1987) some of the coals used for combustion from the San Juan Basin-Range and from the Powder River Region Range have a sulphur content of up to 3.5 wt. %. Most of the coals in the Illinois basin are known to be high-sulphur coals, with sulphur contents of more than 3 wt. %, which limits its use as a fuel source. Inorganic forms of sulphur (mainly pyrite) are usually the predominant forms of sulphur present in the South African coals used in the Sasol Synfuels plant in Secunda. There is also organically associated sulphur that is found within the organic moiety. Trace amounts of sulphur occur as sulphate sulphur on the coal surface.
When reflecting on the fate of sulphur in a typical fixed bed gasification circuit (such as at Sasol), the sulphur is present in the coal on route to the coal preparation plant, where a suitable particle size distribution of the feed coal delivered to the gasification plant is prepared. From the coal preparation plant, the fine coal (<6 mm) is transported to the steam plant and the coarser material (>6 mm) fed to gasification. In the gasifier, the sulphur contained in the coal is exposed to different temperatures as it passes through the various reaction zones. The gaseous sulphur enters the raw gas as H2S (since the top half of the gasifier operates under reducing conditions, and the bottom half under oxidizing conditions), and the H2S is transported through the gas clean-up and gas cooling processes. The H2S is then stripped from the raw gas at Rectisol and sent to the elemental sulphur recovery plant for saleable sulphur product production. Small amounts of the sulphur end up in the gasifier ash where it is trapped in the mineral matter in the ash. Essentially, 99% of the sulphur entering the gasifier reports to the gas phase as H2S during fixed-bed gasification (Skhonde, P. 2009).
The United States (as example) has large reserves of high-sulphur, caking Eastern bituminous coal. Because of the restrictive environmental emission requirement, this coal cannot be used directly for generation of power, unless the station is equipped with sulphur dioxide scrubbers, which raises the cost of power. One approach to meet the environmental emission requirement and to maintain or improve the overall power generation efficiency is the development of hot gas clean-up systems to remove sulphur and particulate contaminants from the fuel gas, thereby eliminating the efficiency losses associated with the cold gas clean-up methods such as wet scrubbing. Simplicity, efficiency, and cost containment are all integral parts of the development of the sulphur removal systems. An attractive system for coal gasification is one in which high-sulphur coal is gasified; sulphur in the coal is retained within the gasifier with ash, eliminating the need for gas clean-up; gas is available for use without first cooling it to preserve the sensible energy; and water vapour is retained in the product gas to make a substantial contribution in the combined-cycle power output. These criteria can be met through the use of a calcium-based sorbent such as limestone or dolomite directly in the fluidized-bed gasifier, which acts as both a catalyst for the gasification reactions and captures sulphur as calcium sulphate (Abbasian, J. 1990).
The Institute of Gas Technology (IGT) has already developed the U-GAS Process to produce fuel gas from coal. The U-GAS process uses a single-stage fluidized-bed reactor to efficiently convert any type of coal, either run-of-mine or washed, into low- or medium-Btu fuel gas that can be used in industrial plants or utility power plants (Abbasian, J. 1990). The process has been developed during 10 years of testing in a 30 tons of coal per day capacity pilot plant located in Chicago and is currently being commercialized. In a new configuration of the U-GAS process, the U-GAS One-Step Desulfurization Process, limestone or dolomite is fed into the coal gasifier to capture and remove sulphur compounds from the fuel gas within the gasifier. Under the reducing conditions of the gasifier, limestone reacts with sulphur compounds to significantly reduce the sulphur content of the fuel gas. Researchers in the field of chemical kinetics of limestone/dolomite reactions with hydrogen sulphide (Rehmat, A et al. 1987; Chang, E et al. 1984; Keairns, D et al. 1976; Borgwardt, R et al. 1984; Pell, M. 1971; Squires, A et al. 1971; Freund, M. 1984; Ruth, L et al. 1972; Kamath, V et al. 1981; Roache, N. 1984 and Abbasian, M et al. 1990) have already verified the potential use of these sorbents for sulphur capture. The reaction of calcined limestone/dolomite is very rapid, and the reaction almost approaches equilibrium. On that basis, it is possible to capture substantial quantities of sulphur and discharge it with the ash (Jones, F. L et al. 1985). Based on equilibrium considerations, it is feasible to remove up to 90% sulphur using this process. No work has been conducted (as far as the Applicant is aware) of in-situ sulphur capturing in a fixed-bed gasifier operating on lump coal particles (>10 mm-100 mm).
Much lab research has been conducted in the field of catalytic gasification, where small coal particles (<1 mm) are gasified (CO2, H2O) in the presence of alkali metals, and have shown that gasification reactivity can be significantly enhanced at temperatures ranging from 800-1000° C. Exxon Research and Engineering Company developed a catalytic coal gasification (CCG) process in the 1970's (More, P et al. 2012). The pilot plant operated at 700° C. and 34 bar, with a 1 ton/day coal throughput. An average particle size of 2.4 mm was used, with a catalyst loading of K2CO3 of between 10-20 wt. %. The coal was fed to a catalyst mixer, where an aqueous catalyst solution was added to the coal. The impregnated coal was dried with an air/flue gas mixture, after which it was fed to a fluidised bed gasifier. More recent research by Nel et. al., (Nel S et al. 2013)] has also shown that the addition of an impregnated K2CO3 catalyst (1% loading) to 10 mm coal particles of R.O.M. Highveld coal lowered the activation energy during reactivity testing when compared to raw coal (Rehmat, A et al. 1987). The challenge remains to increase the catalyst loading to large coal particles. Botha (Botha, A. 2012) successfully demonstrated that the addition of a catalyst (1%, 3%, 5% K2CO3 addition) can be incorporated into a discard Highveld fine coal agglomerate mixture (10 mm pellet) by physical mixing in order to improve CO2 gasification reactivity measured at high temperatures (900-1000° C.). It was concluded that the CO2 gasification reactivity could at least be doubled in this catalysed system when compared to raw coal.
Coal utilisation has also led to rising concerns about CO2 emissions causing global warming. The use of biomass is considered to be renewable and assists in reducing CO2 emissions compared with coal, because biomass is suggested to be CO2 neutral with regard to the greenhouse gas balance (Uson, S et al. 2004; Zhu, W et al. 2008; Biagini, E et al. 2002 and Bonobe, T et al. 2008). The gasification of fuels often occurs primarily through two overlapping stages: pyrolysis and conversion of the char residue (Ciferno, J. P et al 2002). The knowledge of pyrolysis characteristics could be important for understanding better thermochemical conversion of biomass (Yang H, et al. 2007). One of the important features of biomass is the high content of alkali metal in some of the biomass material. Alkali metals, such as potassium, are found to reduce the coal's ash melting point and they are considered to influence the thermochemical conversion processes (Keown, D. M et al. 2005). The biomass metal salts retained after charring could be used as a cheap catalyst during the co-processing of coal (Raveendran, K et al. 1998 and Zolin, A et al. 2001)). Keown et al. stated that these metal salts tend to volatilise during pyrolysis. However, studies done by Nielsen et al. (Nielsen, H. P et al. 200) showed that the volatilisation of these metal salt species from biomass may also cause problems during thermochemical conversion (e.g. slagging and fouling). Currently, particular interest is shown in co-utilization of coal and biomass to produce synthesis gas via the gasification process. The continuous supply of biomass can be challenging if biomass were used alone for thermochemical processes. For instance, poor weather conditions may affect biomass crop supply, storage of biomass (prevention from degradation), grindability, and costs to deliver the crops could increase depending on how far it is from the biomass power plants (Collot, A. G et al. 1999). Coal and biomass can be co-utilised in such a way as to optimize the gas production during thermal treatment.
Thermochemical conversion can be carried out at high temperatures in order to reduce tar formation, and improve the quality and quantity of the product gas formed (Kumabe, K et al. 2007). The resulting synthesis gas is a combination of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), carbonyl sulphide (COS), water (H2O), methane (CH4), nitrogen (N2), higher hydrocarbons (C2+), hydrogen sulphide (H2S), hydrogen cyanide (HCN) and other low molecular mass products. There is limited research being performed to take full advantage of the catalytic properties of inherent alkaline compounds in biomass during the co-pyrolysis process. Most existing studies focus on fast pyrolysis of biomass to produce bio-oil, possible synergetic effects that rise from coal/biomass use during co-gasification, and increasing char reactivity (Moghtaderi, B et al. 2004; Kumabe, K et al. 2007 and De Jong, W et al. 1999). A comprehensive understanding of gaseous products formed during pyrolysis due to co-utilisation is required. The study of thermal treatment of coal, biomass, and coal-biomass blends could provide important information on how to elucidate industrial problems such as clogging of filters, catalyst poisoning, hot-corrosion, erosion, and gas emissions (Gray, D et al. 1996)
The gasification and co-gasification of different types of biomass has been extensively investigated (Tremel, A et al. 2012; Xie, Q et al. 2014; Kaewpanha, M et al. 2014 and Yang, K et al. 2013). The main components of biomass are cellulose, hemicellulose, lignin, extractives, water and mineral matter (alkali and alkali earth metals) (Pereira, H. 1998)). The composition of the biomass used has a significant and direct influence on product distribution from pyrolysis and gasification (Lv, D et al. 2010). Cellulose is associated with fast pyrolysis rates while lignin is said to slow down pyrolysis. Song and co-workers (Song, Y et al. 2013) showed that the addition of biomass to coal enabled the lowering of the gasification temperature while still maintaining the required H2/CO ratio.
One of the main drawbacks of biomass gasification and co-gasification with coal is the formation of heavy tars (Anis, S et al. 2011; Li, X et al. 2009 and Torres, W et al. 2007). Some biomass feedstock with high ash content can cause problems in existing gasifiers due to relatively low ash melting temperatures, high alkali concentrations (Na2O and K2O) and the tendency for slagging, fouling and agglomeration (Yadav, V et al. 2013) that has a negative impact on gasification efficiencies. Pyrolysis and/or gasification of biomass require a high heating rate, and thus pulverisation of the biomass to small particle sizes is required.
Carbonization (also referred to as torrefaction or liquefaction) is a mild thermochemical treatment (200-400° C.) that removes moisture and organic acids from the biomass producing a coal like substance as solid product as well as bio-oil and biogas. The main aim of liquefaction is to reduce a material of low density and energy value into a stable product with high energy density and carbon content ready for use in combustion and gasification processes (Chen, P et al. 2009 and Bridgman, T. G et al. 2008). Liquefaction has the highest net energy gain for biomass conversion to solid, liquid and gas products compared to pyrolysis and gasification (Khoo, H. H et al. 2013).
There is a gap in the literature regarding the combustion and gasification properties of biochar. However, biochar requires less energy for size reduction, has an improved O/C ratio compared to biomass and increases the H2, CH4 and CO content of the gas phase during pyrolysis while lowering the formation of CO2 (Ren, S et al. 2013). Biochar produced through liquefaction from agricultural waste such as sunflower husks (Piyo, N. 2014) was shown to have approximately half the ash content of fine coal (20-30%) Song and co-workers (Song, Y et al. 2013) found that dry gas yield, cold gas efficiency and carbon conversion efficiency increased with an increase in torrefied biomass to coal ratio during gasification.
Biomass material contains significantly higher amounts of alkali (Na and K) and alkali earth (Ca and Mg) metals (AAEM) than most fossil fuels utilised in pyrolysis and gasification (Jiang, L et al. 2012). Raveendan et al. 1995 showed that AAEM present in biomass ash has a significant influence on the pyrolysis characteristics and product distribution during biomass pyrolysis and gasification. Although high concentrations of AAEM in biomass are associated with increased slagging and agglomeration (Knudsen, J. N et al. 2004 and Xiang, F et al. 2012) during biomass gasification, it is also associated with increased reactivity (Duman, G et al. 2014 and Memanova, V et al. 2014). AAEM is said to have a catalytic effect due to interactions with cellulose and the lignin component in the biomass that effect the decarboxylation, decarbonylation and de-esterification reactions taking place during pyrolysis leading to increased CO and CO2 formation at temperatures above 400° C.
Kaewpanha et al. 2014 also showed that alkali earth metals in biomass can increase H2 and CO2 content in the product gas from gasification. Generally, the presence of AAEM in biomass increases char and gas yield while decreasing bio-oil yield (Aho, A et al. 2013). Large amounts of AAEM (50-70%) are lost to the gas phase during pyrolysis above 400° C. and at high heating rates (>10 K·min-1), thus producing chars with reduced amounts of AAEM during gasification, and thus a reduced influence on reactivity. Treatment temperatures during liquefaction are sufficiently low to retain AAEM in the biomass in the biochar (Xue, G et al. 2014)
Whilst Highveld fine discard coal beneficiation has been studied in the past, it has never been implemented for particles <212 micron (EUBA. 2007).
There is thus a clear need in the art to explore the agglomeration of fine discard coal for use in applications, such as fixed bed gasification technologies, thereby affording a reduction in sulphur emissions and other gaseous pollutants concomitant with increased reactivity rates in a manner that overcomes or at least mitigates the shortcomings associated with the prior art.
Furthermore, the current invention provides a solution within the existing gasification island to assist companies in avoiding the need to integrate highly capital-intensive fine coal gasification technologies with clean up infrastructure.